Anonymous lifetime in function return type

Question

I was looking at the source code of Rust, and found this function.

pub fn tokenize(input: &str) -> impl Iterator<Item = Token> + '_ {
    let mut cursor = Cursor::new(input);
    std::iter::from_fn(move || {
        if cursor.is_eof() {
            None
        } else {
            cursor.reset_len_consumed();
            Some(cursor.advance_token())
        }
    })
}

I understand that '_ refers to an anonymous lifetime, but I'm not sure what it means in this context. Would love to get some clarification on this. Thanks.

Answer 1

It's a shorthand for

pub fn tokenize<'a>(input: &'a str) -> impl Iterator<Item = Token> + 'a { ... }

Rust sometimes allows you to avoid declaring lifetimes when the declaration is unambiguous:

fn foo(input: &'a str) -> &'a str { ... }
// same as
fn foo(input: &str) -> &str { ... }

But in the code above, the return type is not a reference, so this shorthand can't be used. '_ is a syntactic sugar for this case.

But why can't the '_ be eluded, too? Here's the answer from RFC 2115: argument_lifetimes:

The '_ marker makes clear to the reader that borrowing is happening, which might not otherwise be clear.